Many devices that employ magnetic fields have heretofore been encumbered by massive solenoids with their equally bulky power supplies. Thus, there has been increasing interest in the application of permanent magnet structures for such uses as electron beam focusing and biasing fields. The current demand for compact, strong static magnetic field sources that reuire no electric power supplies has created needs for permanent magnet structures of unusual form. A number of configurations have been designed and developed for electron beam guidance in millimeter-wave microwave tubes of various types; for dc biasing fields in millimeter-wave filters, circulators, isolators, striplines, and so on. Especially promising for such purposes is the configuration based upon the hollow cylindrical flux source (HCFS) principal described by K. Halbach in "Proceedings of the Eighth International Workshop on Rare Earth Cobalt Permanent Magnets", Univ. of Dayton, Dayton, Ohio, 1985 (pp. 123-136). An HCFS, sometimes called a "magic ring" is a cylindrical permanent magnet shell which produces an intense magnetic field that is more or less constant in magnitude within the cylindrical cavity. The field is perpendicular to the axis of the cylinder and, furthermore, the field strength can be greater than the remanence of the magnetic material from which the ring is made.
The ideal magic ring is an infinitely long, annular cylindrical shell which produces an intense magnetic field in its interior working space. The direction of the magnetic field in the working space interior is perpendicular to the long axis of the cylinder. The aforementioned Halbach publication discloses a structure with an octagonal cross section which closely approximates the performance and field configuration of an ideal magic ring (which has a circular cross section). In both the ideal and Halbach configurations, no magnetic flux extends to the exterior of the ring structure (except at the ends of a finite cylinder). The term "magic ring" as used herein encompasses not only the ideal cylindrical structure with a circular cross section, but also an octagonal, sixteen-sided, thirty-two sided and even higher order polygonal-sided structures which approximated the ideal magic ring.
Fabrication of complex magnetic structures such as the magic ring has been facilitated by the advent of magnetically "hard" materials, i.e., magnetic materials which maintain their full magnetization against fields larger than their B coercivities. Examples of magnetically hard materials are neodymium iron boride (Nd.sub.2 Fe.sub.14 B), samarium cobalt (SmCo.sub.5), platinum cobalt (PtCo.sub.5), and a samarium cobalt alloy (Sm.sub.2 (CoT).sub.17, where T represents a transition metal) together with selected ferrites. These materials may be pressed into various desired shapes and magnetized in a variety of desired orientations using techniques skilled to those known in the art.
However, there is at present no known way to orient and magnetize an annual cylinder in the continuous manner required by the ideal magic ring structure. Fortunately, good approximations are attainable in practice. A sixteen-sided magic ring produces an interior magnetic field which is equal to approximately 98 percent of the field produced by the ideal structure. A coarser eight-sided magic ring still produces an interior field that is approximately 92 percent of the continuous ideal. Nevertheless, the manufacture of numerous magic ring segments, each with a different magnetic orientation is a costly procedure. Although the eight segments of an eight-sided (octagonal) magic ring are geometrically identical, they must be aligned in four different directions, necessitating either four separate dies or some type of indexing procedure that would orient the same die in four different directions. The manufacturing problem, of course, becomes more serious when closer approximations to the ideal structure are sought by an increase in the number of segments.
Those concerned with the development of novel magnetic structures and especially with uses for the magic ring have continuously sought easier, cheaper, and simpler methods for its manufacturer.